1. Technical Field
The present disclosure refers to the field of the control of electric motors and particularly of triphase electric motors.
2. Description of the Related Art
In the field of the triphase electric motors control, it is known the field oriented control (FOC) technique. Such technique typically uses a pulse width modulation (PWM) known as space vector modulation (SVM), that induces sinusoidal voltages and currents in the motor phases which uses a precise measure of the angular position of the rotor and of the motor phase currents themselves.
The motor phase currents are measured by suitable sensors and the obtained measure signals are converted into digital signals by analog-to-digital converters. The phase currents, initially represented by a stationary three-dimensional system (A, B, C), are represented, by the Clarke and Park transforms, in a rotating bi-dimensional reference system (d, q), integral with the rotor angular position.
In such rotating reference system (d, q), assuming a steady condition and that the control generated voltages are perfectly sinusoidal, the direct axis component Id and the quadrature component Iq of the motor stator currents are constant so that they can be controlled by a proportional-integral PI controller.
The motor is driven by a PWM-driven inverter which allows to induce in the motor phases a time-varying voltage between the supply voltage and the ground.
It has been observed that the use of the inverter intrinsically generates voltage harmonics overlapped to the fundamental voltage component. Such voltage harmonics generate higher order current harmonics, which can introduce an error in the current fundamental component measure and substantially reduce the control accuracy.
Therefore, it is desirable that the current measures do not contain the contribution of the higher order current harmonics generated by the inverter.
In the article Richardson J.: “Implementation of a PWM Regular Sampling Strategy for AC Drives”, IEEE Power Electronics Specialist Conference Report, 1989, pages 649-656, it is shown that the contribution of the higher order current harmonics overlapped to the current fundamental component is zero when the currents are measured at the instant t=0 or at the instant t=T/2 of the PWM modulation period T.
It is to be observed that when the currents are measured at instants different from the above-mentioned ones, the higher order current harmonics contribution, in a steady condition, causes a distortion of the measured current behavior. Such distortion of the measured current causes an oscillation of the values of the currents Id and Iq entering the controllers PI and therefore a control error increase, defined as the absolute value of the difference between the reference and the controller input.
The Applicant has further noted that, with real applications, sometimes the motor phase currents are filtered, before being digitally converted, by low-pass filters to eliminate the noise affecting the measure accuracy.
However, the use of the filters, even though it reduces the noise contribution, introduces a time delay of the signal to be measured, causing, as a result, the impossibility to measure the currents at the instants in which is zero the contribution of the higher order harmonics.
The document of Seung-Ho Song et al., “Current Measurements in Digitally Controlled AC Drives”, IEEE Industry Applications Magazine July/August 2000, pages 51-62, discusses the effects due to the use of low-pass filters in a field oriented control of an inverter-piloted apparatus.